Infolded Basal Plasma Membranes Found in Epithelia

One of the most unexpected and extraordinary cytological discoveries of electron microscopy was the observation of Sj6strand and Rhodin (9) and Rhodin (8) that the basal cell surface of the proximal tubule of the kidney is extensively and deeply infolded into the cytoplasmic mass. These folds penetrate between mitochondria and form multiple lateral anastomoses. Pease (6, 7) studied other portions of the nephron, and indicated that the system is most elaborately developed in the distal tubule, and is even present to a moderate extent in the collecting tubule. Pease (6, 7) presented evidence that the osmiophilic lines, which one observes penetrating the mass of the cell, correspond in fact to the diffusion barrier of the cell surface. The narrow gap (110 A.U., Sj6strand and Rhodin (9)) normally present between osmiophilic lines is thus a potential extracellular space. The morphology of the system suggests that it is a device for increasing the surface area in the basal region of a cell. Here, where there are basement membranes and where epithelial cells must be anchored, there is no possibility of developing anything corresponding to a brush border. The problem seemingly is solved in this different, but perhaps equally effective manner. I t seemed important to establish whether or not the system of basal surface infolding is a common specialization, or whether it is a particular feature of the kidney. Although suggestions of it have been incidentally reported in other organs, it has not been systematically searched for, or been the principal object of study. Since an increase in the cell surface might be particularly important in water transport mechanisms, epithelia of the choroid plexus, the ciliary body, and the submaxillary gland have been examined with the possibility of basal specialization in mind. This paper demonstrates that the basal surfaces of all these epithelia are folded in manners reminiscent of the kidney. Specific differences in the pattern of folding suggest independent specialization. * This work has been supported in part by grants from the United States Public Health Service, and from the Los Angeles Heart Association. 203